As environmental concerns have led to increasingly strict regulation of engine emissions by governmental agencies, reduction of nitrogen-oxygen compounds (NOx) in exhaust emissions from internal combustion engines has become increasingly important. Current indications are that this trend will continue.
Future emission levels of diesel engines will have to be reduced in order to meet Environmental Protection Agency (EPA) regulated levels. In the past, the emission levels of U.S. diesel engines have been regulated according to the EPA using the Federal Test Procedure (FTP) cycle, with a subset of more restrictive emission standards for California via the California Air Resources Board (CARB). For example, the Tier II emission standards, which are being considered for 2004, are 50% lower than the Tier I standards. Car and light truck emissions are measured over the FTP 75 test and expressed in gm/ml. Proposed Ultra-Low Emissions Vehicle (ULEV) emission levels for light-duty vehicles up to model year 2004 are 0.2 gm/ml NOx and 0.08 gm/ml particulate matter (PM). Beginning with the 2004 model year, all light-duty Low Emission Vehicles (LEVs) and ULEVs in California would have to meet a 0.05 gm/ml NOx standard to be phased in over a three year period. In addition to the NOx standard, a full useful life PM standard of 0.01 gm/ml would also have to be met.
Traditional methods of in-cylinder emission reduction techniques such as exhaust gas recirculation (EGR) and injection rate shaping by themselves will not be able to achieve these low emission levels required by the standard. Aftertreatment technologies will have to be used, and will have to be further developed in order to meet the future low emission requirements of the diesel engine.
Some promising aftertreatment technologies to meet future NOx emission standards include lean NOx catalysts, Selective Catalytic Reduction (SCR) catalysts, and Plasma Assisted Catalytic Reduction (PACR). Current lean NOx catalyst technologies will result in the reduction of engine out NOx emissions in the range of 10 to 30 percent for typical conditions. Although a promising technology, SCR catalyst systems require an additional reducing agent (aqueous urea) that must be stored in a separate tank, which opens issues of effective temperature range of storage (to eliminate freezing) as well as distribution systems that must be constructed for practical use of this technology. PACR is similar to lean NOx in terms of reduction efficiency but is more expensive due to plasma generator. These technologies, therefore, have limitations which may prevent their use in achieving the new emissions requirements.
NOx adsorber catalysts have the potential for great NOx emission reduction (60–90%). The NOx adsorber is one of the most promising NOx reduction technologies. During lean-burn operation of the engine, the trap adsorbs nitrogen oxide in the form of stable nitrates. Under stoiciometric or rich conditions, the nitrate is thermodynamically unstable and the stored nitrogen oxides are released and subsequently catalytically reduced. Therefore, the operation cycle alternates between lean and rich conditions around the catalyst. During lean operation the catalyst stores the NOx and during rich operation the NOx is released and reduced to N2. However, to make the conditions around the catalyst rich, a significant amount of hydrocarbon (HC) needs to be injected. The amount of HC required for reduction is only a small fraction of the total hydrocarbon injected, resulting in a significant fuel penalty. If the HC required to make conditions rich can be reduced, the fuel penalty can be brought down substantially.
An additional problem is the need for a diesel oxidation catalyst downstream from the NOx adsorber. The diesel oxidation catalyst oxidizes any unburned hydrocarbon that slips through the adsorber before the exhaust gases are released to the atmosphere. The need for a diesel oxidation catalyst negatively affects system cost and system package size.
Furthermore, some diesel engines include a catalytic soot filter to trap the soot generated by the engine. This soot is carcinogenic to living beings. Such catalytic soot filters often become clogged with the trapped particulate matter owing to the fact that they require high temperatures to regenerate. It is difficult to attain these high temperatures in the engine exhaust stream at low loads.
There is therefore a need for an engine aftertreatment system employing an NOx adsorber which reduces the fuel penalty associated with these devices, allows for regeneration of the soot filter, even at low loads, and reduces the system cost and package size. The present invention is directed toward meeting this need.